Physicists, what is the "Hall effect"

What does a hall effect sensor do? How does it work?

http://www.eeel.nist.gov/812/hall.html

Everything (and more) that you ever wanted to know about the Hall effect.

on edit: disclaimer-- I'm not a physicist, so I'll leave the interpretation to you....

:-) I was hoping someone could explain in straight english what
a hall effect sensor does in motion detection.

It seems by the link you made, that it is only good for detecting
transverse voltage in substraits..? Yet i heard a rumor that a hall
effect sensor can detect the motion of a ferrous object.

So is this true. Can i hold up a hall effect sensor and "see" if a
wrench is moved in front of the sensor? I'm drowning in the gobleygook
of electrical engeineering... and though the site has lots of info,
it is just like what i've been googling through.

Have you ever used a hall effect sensor for motion detection?

is a potential difference (voltage) that is set up across a semiconductor when it is exposed to a magentic field.

Link: http://www.willow.co.uk/html/hall_effect.html

They say things like "high repeatability"
and "zero speed"

Do you think they only use these for semiconductor testing?

to achieve whatever its goal is, and not that its purpose is explicitly to measure the Hall Effect. Clicking on one of the devices seems to tell me that it is a position sensor. I have little idea how the Hall Effect might be employed in a position sensor, but then my degree was in physics rather than engineering!

On edit: I am almost certain that the sensor will have a semiconductor inside it, and the device will measure the changes in voltage across the semiconductor as a result of changes in a magnetic field. How the magnetic field is changed, your guess is as good as mine, although I imagine if you moved a metal object around near the sensor that would probably cause a change in a electromagnetic field if one was being created by the sensor itself; by measuring the resultant change in voltage in the semiconductor the sensor might be able to give you information about what is going on outside......? Maybe?

:shrug:

...experienced by a conductor (or semiconductor) that is exposed to a magnetic field.